FIG. 1 illustrates ten cells C1-C10 in a typical cellular mobile radio communication system. Normally a cellular mobile radio system would be implemented with more than ten cells. However, for the purposes of simplicity, the present invention can be explained using the simplified representation illustrated in FIG. 1. For each cell, C1-C10, there is a base station B1-B10 with the same reference number as the corresponding cell. FIG. 1 illustrates the base stations as situated in the vicinity of the cell center and having omni-directional antennas. FIG. 1 also illustrates nine mobile stations M1-M9 which are movable within a cell and from one cell to another. In a typical cellular radio communication system, there would normally be more than nine cellular mobile stations. In fact, there are typically many times the number of mobile stations as there are base stations. However, for the purposes of explaining the present invention, the reduced number of mobile stations is sufficient.
Also illustrated in FIG. 1 is a mobile switching center MSC. The mobile switching center MSC illustrated in FIG. 1 is connected to all ten base stations B1-B10 by cables. The mobile switching center MSC is also connected by cables to a fixed switch telephone network or similar fixed network. All cables from the mobile switching center MSC to the base stations B1-B10 and the cables to the fixed network are not illustrated.
In addition to the mobile switching center MSC illustrated, there may be additional mobile switching centers connected by cables to base stations other than those illustrated in FIG. 1. Instead of cables, other means, for example, fixed radio links, may also be used to connect base stations to mobile switching centers. The mobile switching center MSC, the base stations, and the mobile stations are all computer controlled.
Current digital cellular systems employ base stations which separate mobile stations using time and frequency orthogonality. Signals from a mobile propagate to a base station wherein the signals are received in a single or sometimes a plurality of antenna elements to gain diversity effects. The receiver signal processing uses the time and frequency orthogonality to separate signals from different users. Sometimes, it is desirable to use a plurality of directional antennas or an antenna array to communicate with mobile stations. Use of directional antennas can reduce interference and increase coverage and the number of users. The use of antenna arrays requires some type of beamforming. The beamforming can be implemented in a variety of ways such as digital beamforming, analog beamforming, or by a beamforming matrix, such as a Butler matrix. Analog beamformers steer the beam by introducing a frequency-independent time delay, while digital beamforming usually involves a phase delay that is equivalent to the time delay at an operating frequency.
Several beamforming systems are illustrated in FIGS. 2 and 3. A digital beamforming system usually has a receiver for each element, which down-converts the frequency into I and Q (in-phase and Quadrature) channels for an A/D converter. Real-time beamforming takes place by multiplying these complex pairs of samples by appropriate weights in multiply/accumulate integrated circuits. The array output is formed using a complex signal from n.sup.th channel (V.sub.n), a weighting coefficient (W.sub.n), a steering phaseshift (e), and a correction factor (C.sub.n). Corrections may be necessary for several reasons. These reasons include errors in the position of the element, temperature effects and the difference in behavior between those elements embedded in the array and those near the edge.
Thus, by shaping and directing the narrow antenna beams, a plurality of narrow beams can be used to simultaneously cover a large sector using the same antenna array. The present invention can use an adaptive algorithm for selecting the most feasible functions for the antenna.
The use of directional antennas is, however, sometimes complicated. For example, a base station must be able to transmit broadcast information to a mobile station with an arbitrary position in the cell. However, the cell cannot be made too narrow since this would cause excessive handovers and low trunking efficiency. Hence, there is a desire to have both highly directional antennas and wide lobe antennas in a single cell. One option would be to use multiple antennas in the cell. However, the use of several individual antennas with associated hardware is costly to install and organize. Another obvious solution to one of ordinary skill in the art would be to transmit the common or same information in all of the narrow antenna lobes used in the cell as illustrated in FIG. 4. The disadvantage with this solution is that information from different antenna lobes can sum up to zero, creating undesired nulls or near nulls in the combined antenna pattern 22. As illustrated in FIG. 4, the data to be broadcast is transmitted in all three directional lobes 20. The signals cancel in certain directions and thus deep nulls appear in the combined antenna pattern 22. For example, two lobes with equal amplitude will sum up to zero in a certain direction. As a result, a mobile station with all its scattering points in that direction may suffer from deep fade in signal power, as illustrated in FIG. 5. FIG. 5 illustrates that a mobile station 24 will suffer from very low received power since the mobile station 24 is located in a null in the combined antenna pattern 22. Meanwhile, signals received by a mobile station 26 will have an acceptable received power eventhough the mobile station is also located in a null in the combined antenna pattern 22. In this case, the signal is being reflected off of a building 28 so that the received signal strength is at an acceptable level.
Thus, there is a need for highly directional antennas in order to increase capacity and to improve coverage. There is also a need for antennas with low directivity so that information can be broadcast over the entire cell.